1. Field of the Invention
This invention relates to shaft seals of the type having a polytetrafluoroethylene-lined elastomeric lip.
2. Description of the Prior Art
When a shaft seal having a polytetrafluoroethylene-lined elastomeric lip or a solid polytetrafluoroethylene lip (formed or unformed) is passed over a shaft section having a diameter larger than the lip I.D., the lip takes a set. Thus, when such seals must be passed over a significantly oversized step on a shaft with the requirement that the lip must then retract and seal against another section of the shaft of smaller diameter, the sealing lips thereof take a set resulting in insufficient interference between the lip and the shaft to effect positive sealing.
In the past, efforts to install seals over roughened or enlarged areas of shafts have been directed primarily to the use of special installation methods and tools rather than to new seal designs. The most common practice has been to employ a seal of standard design but stretched to an enlarged I.D. by means of a tapered hollow sleeve. The sleeve covers the enlarged portion on the shaft and the seal is pushed axially past the enlarged portion. The seal is pushed axially inwardly and then the installation sleeve is withdrawn axially outwardly thus permitting the seal lip to return to its original I.D. and thus into sealing engagement with the shaft. However, in some instances the degree of stretch has been excessive and the seal lip has been damaged. Such excessive stretch can result in a situation where the resilience of the seal lip is insufficient to maintain proper sealing contact with the shaft. In more extreme cases, the seal element may be stretched to the point of splitting. In those cases, a new seal must be substituted. Another problem which can be encountered is damage to the seal lip if there are any nicks or rough spots on the installation tool or sleeve.
My U.S. Pat. No. 4,126,316, issued Nov. 21, 1978 and entitled METHOD OF INSTALLING SHAFT SEAL WITH REFORMABLE SHELL describes one way to solve this problem. U.S. Pat. Nos. 2,804,325 and 2,804,324 show prior art designs of polytetrafluoroethylene-lined elastomeric seals. U.S. Pat. Nos. 3,356,376; 3,346,265 and 3,871,669 show prior art designs of elastomeric seals having small flex sections, and U.S. Pat. No. 3,825,271 shows a valve with an air expandable elastic member to vary the valve I.D.
It is an object of the present invention to provide a seal (and the method of installation thereof) with a polytetrafluoroethylene-lined elastomeric lip that can be installed over a larger diameter section of a shaft and can then retract and effect a positive seal against a smaller diameter section of the shaft. It is another object of this invention to provide a combination of such a seal and a shaft having two such different diameter sections.